1. Field of the Invention
The present invention relates generally to consumer device audio power output stages, and more specifically, to a circuit that selects the efficiency of a charge-pump power supply supplying an audio amplifier based on signal level.
2. Background of the Invention
In battery-operated consumer audio devices, the power amplifier output, which is generally utilized to drive a pair of headphones or a speaker, can be the primary consumer of battery power. In devices having a linear power amplifier for the output stage, power is wasted during low signal level outputs, since the voltage drop across the active output transistor plus the output voltage will be equal to the constant power supply rail voltage. Therefore, amplifier topologies such as Class-G and Class-H are desirable for reducing the voltage drop across the output transistor(s) and thereby reducing the power wasted in dissipation by the output transistor(s).
In order to provide a changeable power supply voltage to such a power amplifier, a charge pump power supply may be used, such as that disclosed in the above-incorporated U.S. Patent Application, in which an indication of the signal level at the output of the circuit is used to control the power supply voltage. The above-described topology will raise the efficiency of the audio amplifier, in general, as long as periods of low signal level are present in the audio source. However, the charge pump itself may be a substantial consumer of power, in particular during low audio signal level operation as the relatively constant internal power consumption of the charge pump circuit becomes larger with respect to the power being delivered by the charge pump to the audio amplifier.
Therefore, it would be desirable to provide a charge-pump power supply that supplies power to an audio power amplifier circuit for a consumer audio device, in which the efficiency of the charge pump is improved.